In motorcycles, air is cleaned by an air cleaner element and is guided to a carburetor through an air-intake duct, and an air-fuel mixture is generated in the carburetor and is fed to a cylinder of an engine (see e.g., Japanese Patent No. 2578094). Exemplary air-intake ducts are an air-intake duct having a passage inner diameter that does not substantially vary in an axial direction thereof, and a funnel-shaped air-intake duct having a passage inner diameter that increases toward an air cleaner (see Japanese Laid-Open Patent Application Publication No. Hei. 9-21354). The funnel-shaped air-intake duct includes a pipe-shaped portion that is coupled to a carburetor and has a passage inner diameter that does not substantially vary, and a chamber portion that extends continuously from the pipe-shaped portion so as to increase an inner diameter and is coupled to an air cleaner element.
In the air-intake duct, the pipe-shaped portion is required to have a length sufficient to inhibit occurrence of a turbulent flow to enable air taken in to be supplied smoothly to the engine, whereas the chamber portion is required to have a large volume to reduce an abrupt negative pressure that tends to be generated in the interior of the air-intake duct to thereby improve, for example, responsiveness to an acceleration operation, during acceleration of the motorcycle.
Since a number of components are mounted in a limited space of the motorcycle, the distance between the carburetor and the air cleaner is small, and correspondingly, the whole length of the air-intake duct disposed therebetween is limited. In addition, since a frame member, a rear suspension, etc. are arranged near a space in which the air-intake duct is disposed, the shape of the air-intake duct is limited. Under these conditions, if the length of the pipe-shaped portion is made sufficiently large, then the volume of the chamber portion continuous therewith becomes small, whereas if the volume of the chamber portion is made sufficiently large, then the length of the pipe-shaped portion becomes small. For these reasons, it is difficult to enable both the pipe-shaped portion and the chamber portion to suitably perform their functions.